Warning: the words you are about to read may be stolen

On the literary Dow Jones, glittering prizes go together with charges of plagiarism like bull markets and the Serious Fraud Office. Ian McEwan, the most successful novelist of his generation, has been dogged by imputations of fraudulence. He is not alone. In the past several years plagiarism rows have swirled round Zadie Smith, Jonathan Coe, PD James, Beryl Bainbridge and Graham Swift. McEwan has suffered more than most.

It was alleged his first novel The Cement Garden bore striking similarities to Julian Gloag's 1963 novel Our Mother's House. Next, it was claimed The Comfort of Strangers owed too much to Daphne du Maurier's short story Don't Look Now. Then, last week, up popped the Mail on Sunday to accuse McEwan of copying phrases in his bestselling novel Atonement (2001) from a forgotten memoir published in 1977 by the late Lucilla Andrews, a wartime nurse and queen of the hospital romance.

To anyone familiar with the psychopathology of such cases, it came as no surprise to discover that the MoS broadside coincided with the recent shooting of Atonement, the film, starring Keira Knightley. No surprise, either, that it should prove malicious, hysterical and largely baseless (the few phrases he borrows are simply factual).

McEwan had scrupulously acknowledged Andrews's out-of-print memoir, No Time For Romance, in an author's note. He had even spoken about her on Radio 4. Andrews had recently died of cancer, aged 86, but had taken 'a lighthearted view' of McEwan's debt. If anyone was aggrieved it was her literary agent, and if McEwan was guilty of anything it was of negligence: he had not approached Andrews while she was alive. The MOS journalist, Julia Langdon, denied accusing McEwan of plagiarism. 'I was just making the point that he was discourteous,' she told the New York Times. But there it was: a literary story replete with envy, ill-feeling and implications of foul play.

Plagiarism, one of literature's seven deadly sins, has plagued both the Booker Prize and the Archers. After Yann Martel won the Booker with The Life of Pi, the accusations were misplaced. Martel had already acknowledged that the plot of his novel had been inspired by reading the Brazilian writer Moacyr Scliar.The Archers' plagiarism was more interesting. Here one of the characters described how his grandfather had escaped from Nazi pursuers by hiding under a peasant's skirts - a lift from Gunter Grass's The Tin Drum. Listeners protested but the BBC said it was not 'plagiarism' but 'homage', a traditional defence.

One person's 'homage' is another's 'copyright infringement'. The question of originality in literature is complicated. Until the mid-18th century English writers did not hesitate to borrow from each other's work. Good writers, if they are honest, will acknowledge that when they come across a good thing in someone else's work, either consciously or unconsciously they store it away.

If writers are pickpockets, then Shakespeare is our Fagin, always a 'snapper-up of unconsidered trifles'. Shakespeare did not confine himself to individual lines or phrases. Apart from A Midsummer Night's Dream, his plots were appropriated from other, often classical, sources. This view has been reinforced by imaginative reconstructions of his life, notably Tom Stoppard's Shakespeare in Love. One irony of Stoppard's brilliant script: no sooner had the film won several well-deserved Oscars than it, too, was subjected to accusations of plagiarism.

It was the Romantics, for whom originality of expression was central to literary authenticity, who made plagiarism literature's capital offence. This hasn't stopped all kinds of conscious and unconscious borrowing. No one put it better than the American critic James Atlas, defending novelist David Leavitt against the charge of plagiarising Stephen Spender's World Within World. 'Literature,' Atlas said, 'is theft.' TS Eliot said: 'Immature poets imitate; mature poets steal.'

It has often been observed that the number of basic plots available to the novelist or playwright is tiny. Some say 10, some seven, others as few as five. In this constricted imaginative environment, the likelihood of unconscious repetition is high.

The story is told of an American playwright who had a melodrama staged on Broadway. One day his tailor claimed the play had been stolen from a play that he - the tailor - had been working on. When the playwright questioned the tailor, the man said his play was also about a man accused of a murder he didn't commit. Whereupon the playwright took the tailor up and down Broadwayand pointed out that there were several melodramas running with this theme. 'But they've all stolen my play,' replied the tailor.

This, by the way, is not my story. I pinched it from Performing Flea by PG Wodehouse, a masterly book about the craft of authorship.

Even when writers believe their work to be original, they are often exploring an archetypal story. In considering the sin of plagiarism, there's a distinction to be drawn between copying and reinterpreting old stories. In contemporary fiction there are plenty of examples of the latter. Jane Smiley's A Thousand Acres is a reworking of King Lear, for example, and Peter Benchley's bestseller Jaws has the same plot as Beowulf

In the world of books we are still Romantics. We prize originality above everything and place a high value on novelty of expression. But the more we follow Ezra Pound's injunction to 'Make It New', the more we are brought face to face with the wisdom of Isaiah that 'there is nothing new under the sun'.